Severe combined immune deficiency (SCID) is a rare genetic syndrome marked by the absence of T and B lymphocyte function (1). In the past 10 years, the genes responsible for most forms of SCID have been identified, cloned and their function and expression characterized. Defects of genes involved in lymphoid metabolism, cytokine responses, immunoreceptor expression or signaling, and antigen presentation have been described. The most common form of SCID is X-linked SCID (X-SOlD), which is due to mutations in the common gamma (gammac) subunit of the receptors for IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21 (2-10). An autosomal recessive form of SCID with a similar thymopoietic defect as seen in X-SCID is due to deficiency of the IL-7 receptor (IL-7Ralpha subunit, which together with the gammac comprises a functional IL-7R (11,12). In both X-SCID and IL-7Ralpha deficient SCID, the loss of IL-7R signaling leads to an inability of T lymphoid progenitors (prothymocytes) to survive, proliferate and differentiate (13-19). Failure of prothymocytes to develop also alters the development of the thymic microenvironment, resulting in a disorganized, atretic thymus that may be impaired in its ability to support thymopoiesis (20-22). Recent studies have also shown that the IL-7R is necessary for the maintenance of the mature T lymphocyte compartment. B lymphocytes are generated in human X-SCID and IL-7Ralpha deficiency because the IL-7Ralpha does not appear to be necessary for the development of human B lymphoid progenitors. Normal differentiation of T lymphoyctes from hematopoietic stem cells (HSC) occurs through several phenotypically and anatomically defined intermediate stages of differentiation. Common lymphoid progenitors (CLP) are marrow cells that give rise to T, B, and NK lymphocytes (23). Unlike HSC, CLP are highly proliferative. CLP-derived committed T lymphoid progenitors in the thymus (prothymocytes) that have recently entered the thymus can be phenotypically characterized by cell surface markers and T cell receptor (TCR) gene rearrangements. Common lymphoid progenitors, prothymocytes and immature thymocytes all express IL-7R, as well as the c-kit receptor. Together, IL-7R and c-kit mediated signals are necessary for development of thymocytes. Prothymocytes reciprocally produce cytokines that are necessary of the development of the thymic microenvironment. In mudne allogeneic models, transplanted CLP can more rapidly reconstitute immunity than HSC. Gene therapy for primary immune deficiencies has generally been targeted at primitive hematopoietic cells, ideally HSC. Transduction of HSC with retroviral vectors can lead to the permanent integration of the normal gene into the HSC and all of its progeny. Thus, retroviral-mediated gene transfer into HSC can theoretically correct the defect in all of the known pdmary immune deficiencies, which are restricted to the hematopoietic lineage. The limits of retroviral gene therapy are due to the inefficient transduction of HSC, difficulty in obtaining sufficient numbers of HSC for ex vivo transduction, slow reconstitution of immunity from transplanted HSC, and limited control of gene expression. X-SCID has been a particularly attractive model for retroviral gene therapy because 1) there is a selective advantage to progenitors and mature lymphocytes which express the normal SCID gene, and 2) because the normal gammac gene is expressed in all hematopoietic lineages. The selective advantage of cells expressing normal gammac means that transduction of small numbers of immature progenitor cells may produce clinically significant numbers of mature T lyrnphocytes. The ubiquitous expression of gammac during hematopoiesis obviates the need for lineage-specific expression of the gammac transgene. However, the lack of a selective advantage in the B lineage to transduced cells in X-SCID means that reconstitution of a B cell repertoire may depend on transduction of a large number of HSC. One way to achieve higher levels of HSC transduction would be the use of lentiviral vectors, which unlike retroviral vectors, can transduce non-cycling cells. In contrast to the gammac chain in X-SCID, the IL-7Ralpha is expressed in a lineage-specific manner, i.e., only in lymphoid cells. Because the IL-7Ralpha may have an instructive role in lymphopoiesis, the constitutive expression of IL-7Ralpha resulting from retroviral transduction may alter the development of transduced hematopoietic progenitors. Forexample, there is significant overlap between downstream pathways mediated by the lymphoid-specific IL-7R and the erythroid-specific erythropoietic receptor (EpoR). Thus, ectopic expression of the IL-7R in hematopoietic stem cells, e.g., after retroviral transduction with an IL-7Ralpha vector, may alter the development of non-lymphoid lineages. The use of lentiviral vectors with elements that confer lymphoid-specific expression may overcome this problem.